A pacemaker is an implantable device that delivers electrical stimulation pulses to cardiac tissue to relieve symptoms associated with bradycardia—a condition in which a patient cannot maintain a physiologically acceptable heart rate. Early pacemakers delivered stimulation pulses at regular intervals in order to maintain a predetermined heart rate, which was typically set at a rate deemed to be appropriate for the patient at rest. The predetermined rate was usually set at the time the pacemaker was implanted, although in more advanced devices, the rate could be set remotely by a medical practitioner after implantation.
Early advances in pacemaker technology included the ability to sense intrinsic cardiac activity, i.e., the intracardiac electrogram, or “IEGM” signal. This led to the development of “demand pacemakers,” so named because they deliver stimulation pulses only as needed by the heart. Demand pacemakers are capable of detecting a spontaneous cardiac contraction that occurs within a predetermined time period (commonly referred to as the “escape interval”) following a preceding contraction, whether spontaneous or evoked by the implantable device. When a naturally occurring contraction is detected within the escape interval, a demand pacemaker does not deliver a pacing pulse.
Pacemakers such as those described above proved to be extremely beneficial in that they successfully reduced or eliminated seriously debilitating and potentially lethal effects of bradycardia in many patients. However, since pacemakers are implantable devices, an invasive surgical procedure is required, and many patients who receive pacemakers must undergo several surgical procedures, because pacemakers have a limited life span, due to limited battery life, and require periodic replacement. Of course, it is desirable to minimize the number of surgical procedures that must be performed on a patient to improve safety and reduce costs.
The life span of most pacemakers is dictated by the rate at which their batteries drain. Thus, a substantial effort has been directed toward minimizing the amount of energy used by pacemakers, while ensuring that the devices continue to deliver effective therapy. Demand pacemakers effectively reduce battery drain by delivering pacing pulses only when required. However, each pacing pulse delivered by a demand pacemaker may have a significantly higher energy content than that required for inducing a cardiac contraction. Thus, even after the development of demand pacemakers, there remained an opportunity for further improvements in the area of pacemaker energy utilization.
The minimum amount of electrical stimulation that effectively evokes a cardiac contraction is commonly referred to as a patient's “capture threshold.” Unfortunately, capture threshold varies significantly among patients; therefore, the amount of electrical stimulation provided by a pacemaker cannot be permanently set by the manufacturer. Rather, stimulus parameters must be individually set for each patient immediately after implantation and during subsequent office visits.
Determining a particular patient's capture threshold is a relatively simple procedure when performed during an office visit. Essentially, the medical practitioner can remotely adjust the amount of electrical stimulation downward from a maximum value that is known to elicit a contraction for all patients. Once the amount of electrical stimulation falls below the patient's capture threshold, an ensuing heartbeat is not detected, and the medical practitioner upwardly adjusts the amount of electrical stimulation beyond the last successful level.
Typically, a substantial safety margin is added to the measured capture threshold to ensure that the pacemaker continues to evoke contractions over an extended period of time. The safety margin is necessary because a patient's capture threshold varies over time—sometimes dramatically during the first few months following implantation. However, by adding such a large safety margin, it is almost assured that the pacemaker will be wasting significant amounts of energy during its life span.
In an effort to reduce the amount of energy wasted, pacemakers have been developed that automatically evaluate the patient's capture threshold during normal operation. These devices are also capable of automatically adjusting the amount of electrical stimulation in response to changes to the capture threshold. These features, which in combination are referred to herein as “automatic capture detection”, significantly reduce unnecessary battery drain, because higher energy pacing pulses are delivered only when needed by the patient. Although most of these devices continue to add a safety margin to the measured capture threshold, the safety margin can be greatly reduced, especially when the capture threshold is measured frequently.
Pacemakers that perform automatic capture detection commonly monitor a patient's IEGM signal to determine what pulsing energy level is necessary to evoke a responsive cardiac contraction (“evoked response”). In particular, the pacemaker samples the portion of the patient's IEGM signal corresponding to the evoked response, if any, immediately after a pacing pulse is delivered. The shape of the waveform indicates whether the pacing pulse successfully captured the heart (i.e., whether the pulse caused a corresponding contraction of the heart chamber).
However, known automatic capture detection methods have several drawbacks, particularly relating to signal processing, which have proven difficult to overcome. For example, it is extremely difficult to accurately sense the evoked response immediately after a pacing pulse is delivered, due to the presence of residual electrical effects in the immediate vicinity of the pacing electrodes. These residual effects (commonly known as “polarization”) interfere with the pacemaker's ability to detect an evoked response. Indeed, most pacemakers enter a refractory period immediately after a pacing pulse is delivered, during which time the sensing circuitry is deactivated, for the specific purpose of avoiding undesirable sensing of polarization.
An additional drawback of conventional pacemakers that perform automatic capture detection is detection of an evoked response signal that may vary in its morphology. This variability can be introduced by many physiological factors, including the patient's drug therapy and circadian rhythm. One particular factor that contributes to a large variation in the morphology of the evoked response is the patient's posture and/or level of activity. For example, differences in the morphology of an evoked response signal from a single patient have been observed when the patient is standing, sitting, lying down, at rest, or active (e.g., exercising or other form of moderate or strenuous activity), hereinafter referred to as the “patient state.” Conventional automatic capture detection systems and methods do not consider patient state-induced variability when detecting the evoked response, or when setting the evoked response detection threshold or any other device parameters that are susceptible to such variability.